1. Field of the Invention
The present invention relates to radio frequency identification transponders, and more particularly concerns transponders having an improved low cost configuration.
2. Description of Related Art
RF transponder systems are used to monitor, at a short distance, for the presence of an object associated with a transponder, to identify the object and/or to communicate various types of information about the object back to a receiving station. Such systems typically employ an exciter/reader that transmits an RF excitation signal, and a transponder that is energized by the excitation signal to transmit a signal including an identification code and/or other information back to the exciter/reader. The transponder receives a radio frequency signal, or, more specifically, is energized by the RF magnetic field and forms a response signal that will identify the transponder and which may provide additional information, and then re-transmit a response signal back to the exciter. The exciter includes a receiver that receives the response signal and processes the information it contains.
In one such system, such as a radio frequency identification system made by Hughes Identification Devices, a subsidiary of Hughes Aircraft Company, the exciter/transmitter includes a transmit/receive coil which sends out a radio frequency excitation signal. The same coil also receives the response signal from a transponder. The response is formatted by the transponder as a frequency shift keying (FSK) signal that is received, processed and demodulated by the exciter.
One type of transponder uses an antenna formed on a thin PC circuit board made of a rigid dielectric on which is mounted an integrated circuit transponder chip. Typical applications include transponder tags on clothing airline baggage identification, and security access in general. Transponder cards, which may be the size of a credit card, can be used in many applications where bar codes and magnetic strip tags are presently used.
Devices of this type are of relatively limited range, in the order of between two to three inches up to about thirty inches. Accordingly, performance, largely defined by the range of the system, is a major criterion for further development. Performance is limited by, among other things, the ability of the transponder antenna to clearly receive energy and re-transmit appropriate signals.
In prior coil devices, antenna coil Q and inductance may vary from transponder to transponder because of differences in manufacturing configurations of the antenna coils. This may result in varying and unreliable read range performance. The specific physical position and proximity of coil turns with respect to each other and variation in pressure applied to the coils because of variation of protective cover lamination will also vary reading range performance in prior devices. Wire connections from the coil, made directly to a circuit on board (COB), are subject to breakage because of flexure, vibration, compressive forces and thermal expansion and contractions. In addition, wires that form the turns of the coil itself are subject to breakage for the same reasons. Because of the use of a chip on a circuit board or a printed circuit board, it is difficult, if not impossible, to obtain a sufficiently thin final assembly or one that will meet standards of the International Standard Organization.
Low antenna coil Q often results from use of epoxy glass printed circuit board technology, having relatively large spacing between coils. Such arrangements, accordingly, require more turns for a given inductance. Present designs employing large line and space widths of 5 to 10 mils or more require turns to cover all or most of the available area, resulting in a lower inductance, lower Q, and fewer flux lines encompassed in a given field, all of which result in lower performance in terms of read range. Because prior designs require coil turns to cover substantially the entire area of a small credit card size transponder, the area within and surrounded by the coil that receives magnetic flux lines of a given field is relatively small, thus decreasing read range and decreasing coil Q.
Accordingly, it is an object of the present invention to provide a transponder and method for making a transponder that avoids or minimizes above-mentioned problems.
SUMMARY OF THE INVENTION
In carrying out principles of the present invention in accordance with a preferred embodiment thereof, a thin dielectric substrate is provided, having first and second sides, a die mounting site on the first side, a plurality of bond pads and a first multi-turn flat antenna coil on the first side. A first via extends through the substrate and is connected to an inner end of the first coil, and a second via is formed through the substrate adjacent one of the bond pads. A second multi-turn antenna coil is formed on the second side of the substrate, having a first end at an interior portion of the substrate connected to the first via and having a second end connected to the second via on the second side of the substrate. A protective coating is formed over the first and second sides of the substrate and over the antenna coils, leaving uncovered the die bond site and bond pads. The second via on the first side of the substrate is connected to one of the bond pads. A die is mounted at the die bond site and electrically connected to the bond pads. A nonconductive encapsulation is provided over the die and bond pads and first and second protective laminates are secured to each other, covering both sides of the substrate and protecting both antenna coils die and bond pads.
According to a feature of the invention, a plurality of programming pads is formed on the first side of the substrate with programming leads interconnecting the programming pads and the bond pads.
According to another feature of the invention, an interior slot is formed through the substrate and the protective laminates on opposite sides are bonded to one another through the interior slot and around the perimeter of the substrate.
According to another feature of the invention, the coils formed on the first and second sides of the substrate are formed in relatively narrow perimetrical areas on the first and second sides so as to circumscribe interior portions of the thin substrate, whereby a major portion of the substrate on both first and second sides is free of the antenna coils to provide a large central area for accepting magnetic flux lines.